Buy a rear brake cylinder re-winder $60. However, you can also struggle with a C-clamp and needle nose pliers, pressure - turn, pressure-turn, pressure - turn. The rear pistons rotate as brake pressure is applied which allow them to lock with the parking brake.
You will need a caliper rewinding tool
It requires a special tool to turn and compress the piston in at the same time.
with a large pear of channel locks,or a caliper compressor
Most likely the piston will need to be turned not compressed
Rewind it with a piston rewind tool or, at a pinch, use needle nose pliers.
It requires a special tool to twist the piston back into the caliper.
the caliper piston turns clockwise to compress,the gm w-bodys are known for bad rear calipers. gmjim
If it is the front brake use a "C" clamp to retract piston If it is the rear and the parking brake is used by this caliper piston is threaded and a special tool (not expensive) available at Sears or auto parts to "screw" piston into caliper body
The piston in the rear caliper on the 2008 Grand Caravan is the "screw in" type. A special tool is required to do the job.
Shade tree mechanics use a C-clamp and a flat steel bar to compress the piston. I have also used a large adjustable Channel-Lok-type pliers to compress.
Use a C clamp to push the piston out of the way to remove and also to place back on rotor.
with a c clamp. put a block of wood or your old pad over the piston and tighten with clamp.